introduce floordiv, ban regular div for integers

[charles-cooper]

What I did

implement #2847

(note: wait until after v0.3.4)

How I did it

add FloorDiv node, add typechecker rules

How to verify it

tbd
note:

@external
def foo(x: uint256, y: uint256) -> uint256:
    ret: uint256 = x / y  # banned
    return x // y

@external
def bar(x: decimal, y: decimal) -> decimal:
    ret: decimal = x // y  # banned
    return x / y

Commit message

Commit message for the final, squashed PR. (Optional, but reviewers will appreciate it! Please see our commit message style guide for what we would ideally like to see in a commit message.)

Description for the changelog

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[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (29205ba) 84.99% compared to head (1630151) 85.03%.
Report is 8 commits behind head on master.

Files	Patch %	Lines
vyper/semantics/types/primitives.py	86.20%	3 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2937      +/-   ##
==========================================
+ Coverage   84.99%   85.03%   +0.03%     
==========================================
  Files          92       92              
  Lines       13714    13764      +50     
  Branches     3079     3083       +4     
==========================================
+ Hits        11656    11704      +48     
- Misses       1570     1571       +1     
- Partials      488      489       +1     

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